The present invention relates generally to the nitrile synthesis wherein a mixture of an aliphatic alphamonoolefin such as propylene of isobutylene, ammonia and oxygen are interacted over a solid ammoxidation catalyst at elevated temperature producing an unsaturated aliphatic nitrile such as acrylonitrile from propylene or methacrylonitrile from isobutylene. More specifically, the invention relates to an improvement in the control of such synthesis allowing most accurate control of the input gas stream and the accurate and fast determination of conversion, selectively and yield not only the desired nitrile product but also of unreacted feed materials and/or by-products some of which are accurate telltales or correct or incorrect operation.